Effects of CO2 intrusion on pore structure characteristics of mineral-bearing coal: Implication for CO2 injection pressure

Abstract

CO2 intrusion has a crucial effect on the pore structure of mineral-bearing coal. In this study, we selected long flame coal, lean coal, and anthracite after CO2 adsorption at different pressures and tested the coal samples using X-ray diffraction, mercury intrusion porosimetry, and N2 (77 K) adsorption methods. The tests were conducted to determine the variations in mineral content, pore structure, and fractal characteristics. The results showed that supercritical CO2 had a greater ability to dissolve minerals in coal than that of subcritical CO2. Although the total pore volume and BET specific surface area gradually increased with the increase in CO2 intrusion pressure in coal, the transformation of different pores and partial new pores caused by the dissolution of minerals and the adsorption swelling of coal matrix caused the micro-macropores in the three coal samples to exhibit different trends. The pore surface roughness and pore structure complexity of seepage pore in the long-flame coal after CO2 adsorption increased while those of the lean coal and anthracite decreased. Meanwhile, CO2 intrusion caused the surface of the adsorption pore in coal to become smooth, and the pore structure was more regular, except for the lean coal. A conceptual model of the mineral-bearing coal was developed to describe the relationship between the mineral composition and pore structure induced by CO2 intrusion. These findings help to understand the transformation effect of CO2 on coal seams. Thus, a higher CO2 injection pressure should be used to obtain a larger injection volume and shorter injection time during CO2 storage implementation.